Lucky Starr and the Rings of Saturn, by Isaac Asimov

The Sirians have colonized Titan, saying that the Saturn system was empty so it's theirs now. Earth won't stand for having an enemy base in their solar system, but to force the Sirians out will mean starting a war. Lucky Starr calls an interstellar council and persuades the rest of the solar systems to back Earth. Sirius is forced to withdraw.

I do so love when a problem is solved by people talking in rooms. Really. Asimov is fantastic at making conversations and diplomatic negotiations enthralling.